This invention relates to pressure and flow control means for hydraulic control valves, and relates to such instrumentality for multiple directional control valve assemblies.
Multiple spool control valves generally comprise a plurality of directional control valve sections, each provided with a shiftable, cannelured control spool controlling fluid flow to one or more specific hydraulic motors, sandwiched between inlet and outlet end plate sections having ports connectable with a source of motive fluid and a low pressure reservoir. Open center type assemblies permit continuous flow transversely through the assembly from inlet to outlet when all the spools are in neutral non-operative positions. Upon shifting a control spool to divert the motive fluid to actuate the associate motor, the spool variably shuts off the open center flow.
A serious drawback to such open center assemblies is the variable pressure differential exerted across the shifting spool. As well known in the art, the resulting abrupt change in direction of the flow at this juncture, Bernoulli forces and other factors combine to exert a substantial force tending to shift the spool. With larger gallonage valves operating at sometimes quite high pressure, the forces exerted on the spool make it extremely difficult to control spool movement manually. While this problem is somewhat reduced by utilizing power means other than manual force to move the spool, the same forces caused by these pressure differentials are still present, and control of spool movement remains too unpredictable and non-repeatable to provide precise and effective flow control. Furthermore, the use of power means to shift the spool drastically compounds the complexity, expense, and other undesirable features of flow control valves.
Another problem associated with multiple valve assemblies capable of handling large flow volumes relates to the master relief valve. For economy and convenience, it is highly important that the master relief valve, which must be capable of handling the maximum flow volume of the valve assembly to protect same under all conditions, be of a compact nature capable of being incorporated into the control valve assembly itself. Placement of additional housings onto the valve assembly is not feasible in many instances because of critical space limitations. To function properly, however, the master relief valve is best located as near as possible to the control valves to insure immediate relief of build-up of excessive pressure. Such consideration is also highly important in minimizing extremely high pressure surges of short duration which occur for various reasons and particularly when quickly opening and closing fluid passages by sudden movement of the closing fluid passages by sudden movement of the control valve spools.
A particular type of control valve circuitry that has found wide acceptance in valve assemblies is regenerative circuitry. This is especially necessary in operating a heavily loaded hydraulic motor whose external load is capable of outrunning the fluid flow delivered thereto, resulting in a cavitation condition. Regenerative circuitry alleviates this problem by diverting the fluid flow being exhausted from the overrunning motor back to the inlet side of the motor to supplement inlet flow. While such circuitry is adequate in many applications, in certain situations pressure surges develop in the regenerating exhaust flow that is being carried through the low pressure passages in the control valve assembly. In such instances the sealing members surrounding these low pressure passages are easily destroyed by the pressure surges. The expense involved in attempting to utilize high pressure sealing members for the low pressure return passages has heretofore been found unacceptable, rendering the valve assemblies non-competitive.
Accordingly, it is the primary object of the present invention to provide an open center type control valve having a pressure control member that automatically maintains a substantially constant pressure differential across the control valve spool at the open center passages to provide constant handle forces and accurate metering of the control spool.
A more particular object in accordance with the preceding object is to provide a pressure responsive member intercepting the open center passages downstream of the spool and operable to variably restrict flow through the open center passages, the member being positioned in response to the pressure differential created by the spool in a manner maintaining a constant pressure differential across the latter.
A corollary to the above objects is to provide such a pressure control member in an assembly of stack valves that is operable to maintain a constant pressure differential across any one of the spools that is shifted.
Another object of the invention is to provide a pressure relief valve capable of relieving fluid flow through more than one exhaust passage to present a compact valve capable of being incorporated within the housings of control valve assemblies.
A further object in accordance with the preceding object is to provide a relief valve of the type described that is pilot operated to provide superior valve opening characteristics.
Another important object of the present invention is to provide an assembly for controlling a hydraulic motor subject to large inertial loads, wherein there is provided low pressure regenerating means for diverting fluid displaced from one side of the motor back to the other side to preclude formation of cavitation conditions, and wherein is provided means for relieving instantaneous surge pressures in the low pressure return circuitry to prevent seal destruction.
A more particular object in accordance with the preceding object is to provide means of the type described that may be easily and conveniently incorporated into directional control valve assemblies.
Another broad object of the present invention is to provide an improved end plate section of universal configuration capable of being utilized as either an inlet or outlet port plate for sectional control valve assemblies and which is provided with a transfer passage means selectively communicating fluid inlet and outlet ducts of the valve assembly, the end plate section being configured to permit utilization of various fluid flow control instrumentalities for controlling flow to the transfer passage means.